One square foot of net free vent area for every 300 square feet of attic floor, split half intake at the soffits and half exhaust at the ridge. A 1,600 square foot attic needs 5.33 square feet of net free area — 768 square inches — which is 384 square inches each way.
The Roof Vents Calculator computes the Net Free Area (NFA) of intake and exhaust ventilation required by IRC code for your attic. Standard rule: 1 sq ft of NFA per 150 sq ft of attic floor, split 50/50 between intake (soffit vents) and exhaust (ridge vents). Enter attic square footage and the tool returns the NFA needed plus the count of standard ridge and soffit vent pieces. Balanced ventilation prevents ice dams, mold, and premature shingle failure.
Attic ventilation is sized by the 1/300 rule: one square foot of net free vent area for every 300 square feet of attic floor, split evenly between intake at the soffit and exhaust at the ridge. Where there is no vapour barrier, 1/150 applies.
| Attic floor | 1/300 total NFA | Intake half | Exhaust half | Ridge vent equivalent |
|---|---|---|---|---|
| 800 sq ft | 2.7 sq ft (384 sq in) | 192 sq in | 192 sq in | 11 ft of ridge vent |
| 1,200 sq ft | 4 sq ft (576 sq in) | 288 sq in | 288 sq in | 16 ft of ridge vent |
| 1,600 sq ft | 5.3 sq ft (768 sq in) | 384 sq in | 384 sq in | 21 ft of ridge vent |
| 2,000 sq ft | 6.7 sq ft (960 sq in) | 480 sq in | 480 sq in | 27 ft of ridge vent |
| 2,400 sq ft | 8 sq ft (1,152 sq in) | 576 sq in | 576 sq in | 32 ft of ridge vent |
Ridge vent is rated around 18 square inches of net free area per linear foot; soffit vents vary widely, so read the figure printed on the product.
The attic ventilation calculator works the 1/300 and 1/150 rules through for your attic floor area and converts the result into feet of ridge vent and the number of soffit vents, using the net free area figures printed on the products.
The simplest test is temperature. On a hot day a well-ventilated attic sits within roughly 10 to 20 degrees of the outside air; a poorly ventilated one runs far hotter. In winter the tell is frost on the underside of the sheathing, or ice dams along the eaves, both of which mean warm moist air is reaching the deck and condensing.
If the numbers say you have enough vent area but the attic still runs hot, the problem is nearly always intake. Go into the attic with a torch, look along the eaves, and check you can see daylight through the soffit vents. If insulation has been pushed into the eave, baffles at each rafter bay restore the path.
One square foot of net free vent area per 300 square feet of attic floor when there is a vapour barrier, split half intake and half exhaust. Without a vapour barrier the requirement doubles to 1/150.
Ridge vent gives about 18 square inches of net free area per linear foot. A 1,600 square foot attic needs 5.33 square feet of total net free area, so about 21 feet of ridge vent for the exhaust half.
Yes. If exhaust exceeds intake the ridge vent starts drawing air in at one end and out the other instead of pulling from the soffits, and in the worst case it pulls conditioned air out of the house.
Almost always blocked intake. Insulation pushed into the eaves closes the soffit vents, and a ridge vent with no intake cannot move air. Baffles at the eave fix it.